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'Barn doors' on bandpass subs?

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Lincoln1 View Drop Down
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    Posted: 16 February 2009 at 8:52pm
Hi all, first I'll apologise as I'm new here and didnt know where to post this q. It's not really a newbie one I dont think.
Anyway I have 4 x 6th order 800W 18" bandpass subs (check newbie section for pics). They are around 108dB efficiency which is more than enough for most uses I have, but when outdoors they dont throw as well as w-bins obviously.
However can I clip some barn doors, or sections of 1200mm plywood to the fronts of the cabinets to form a horn and gain some throw and maybe even some spl?


Edited by Lincoln1 - 16 February 2009 at 8:53pm
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Deadbeat View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Deadbeat Quote  Post ReplyReply Direct Link To This Post Posted: 16 February 2009 at 10:00pm
SPL = throw

the bigger your source SPL, the more throw.

Look at the 'X1 barndoor' topic for ideas.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Lincoln1 Quote  Post ReplyReply Direct Link To This Post Posted: 16 February 2009 at 10:12pm
I cant find that thread? please post?
 
I dont know that outright spl means throw. Indoors there is enough bass to make you feel sick, but not from a long distance outdoors, say any more than around 15-20
metres. Basically my subs arent very directional. If you stand beside (or even behind them? WTF?) when outside they still have quite high volume. I find with w-bins they are more directional, hence my question about some panels to direct them outwards, also possibly helping with throw. I dont know whether this will affect the loading of the box design also.
I do like the bandpass sound though as it has no boomy harmonics, so want to expand their use since I already have them and like their sound and volume/efficiency.
 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Deadbeat Quote  Post ReplyReply Direct Link To This Post Posted: 16 February 2009 at 10:23pm
The apparent loss of bass would be due to room modes, etc.

Bass wavelengths are very large, and only with a comparably large radiation area will they begin to be directional. That's one of the reasons a 12" is usually crossed higher than a 15" to a comp - as it is smaller, it beams less at a given frequency as you go up. Granted, a W-bin will have more radiation area than your BPs, but it is nowhere near enough to impart actual dispersion. Think about the sizes of the pressure waves involved and all your subs seem very small indeed. Sound diffracts around small things.

What's probably the case is that the bandpasses aren't actually 108dB 1W/1m.

'(or even behind them? WTF?)'

I don't see what is wrong here.


Edited by Deadbeat - 16 February 2009 at 10:23pm
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Lincoln1 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Lincoln1 Quote  Post ReplyReply Direct Link To This Post Posted: 17 February 2009 at 12:31am

There is no 'problem' as such, just an improvement in direction if possible. Can I take it from your answer that barn doors wont direct the bass more in a forward direction?

Also I am mistaken, I built these bins around 15 years ago and I forgot some of the specs. Back then I used bassbox4. I see that is now called JBL speakershop.
Anyway they are 107-108dB at the peaks, and 105dB at the dip in the middle. Here's the pic with the L18P200N bandpass box I have graphed.
 
Cheers for the reply.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote tb_mike Quote  Post ReplyReply Direct Link To This Post Posted: 21 February 2009 at 8:27am
Unless your boundary is comparable to the wavelengths involved,ie small house sized,not much will change.

 The directivity of the wbins wont be much at 50hz,but at 200hz behind them will sound different to infront - the mouth will have some pattern control over higher octaves since the wavelengths are smaller eg 2.1m(160hz) not 8.5metres(40hz). 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Lincoln1 Quote  Post ReplyReply Direct Link To This Post Posted: 21 February 2009 at 8:11pm
Cool, thanks for that. I cross mine over at 100Hz anyway so there goes that theory of getting direction. I was also asking alot as on 2nd listen outside when I was sober there was pretty reasonable bass at a good distance away. Youd hope so at 105-108dB 1 watt, then 5kW into the 4 subs. The graph is 2 and the other 2 are tuned slightly lower. around 5 Hz from memory.
 
I have also read that article on direction and lobing by Rog Mogale, Bass arrays. So it lines up with your theory of low freg dispersion.
 
Cheers mike, saves me buying some plywood to experiment with horns/doors.


Edited by Lincoln1 - 21 February 2009 at 8:14pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Stumbler Quote  Post ReplyReply Direct Link To This Post Posted: 19 March 2009 at 11:47pm
does not the FK1 Infrahorn give the Infrabass cabinet a big boost in output in the mid 30's and far more directionality and 'throw' tho..?  and that only uses 18mm ply...
not as straight forward as i first imagined..
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Post Options Post Options   Thanks (0) Thanks(0)   Quote sKs01 Quote  Post ReplyReply Direct Link To This Post Posted: 20 March 2009 at 12:01am
theirs deffinatley something to be said for the idea in principle, the efficiency gain is a good one, you cant realy impart directionality onto bass unless you had an exceedingly silly amount of horns,

my only concern over the idea is that bandpass cabs are demanding on a cone as it is loading the cone as it is any more could well just end up with lots of confetti, i think fk1 will have considered this problem and with acsess to oem drivers as well as a large r &d budget they can come up with a solution that wont mince the cones.

just slapping some plywood barn doos on a bandpass cab is not thew same thing but worth a try if the consiquences of it going wrong are within the reach of your pocket, espeialy as the main way to get a design like this to work from an existing cab is a bit trial and error at best.
You're a big man, but you're in bad shape. With me it's a full time job. Now behave yourself.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Rog Quote  Post ReplyReply Direct Link To This Post Posted: 20 March 2009 at 12:50am
Lincoln1, you have 2 problems here.
 
1. The speakershop program has an error when it calculates bandpass boxes. It shows around 6 to 7dB more gain that is really happening. this was corrected with Bass Box Pro. If you really had a bandpass box with that bandwidth that was 108dB at 1w/1m you could be a millionaire.
 
2. Unless the barndoors are made at least as strong and preferably stronger than the bass cab you will lose SPL. For a barndoor to work at low frequencies (<60Hz) requires monumental bracing and weight. A good one should weight more than 80Kg if its going to work. If you just put two sheets of wood on hinges either side, all the energy from the bass bin will go into shaking the sheets of wood and will not radiate into space as well as it would of without the barndoor.
 
Barndoors really have to be an integral part of the enclosure and design. Flapping sheets of wood are just that. Look at how a bass trap works, thay are large sheets of wood or other materials that can move.
 
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Lincoln1 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Lincoln1 Quote  Post ReplyReply Direct Link To This Post Posted: 20 March 2009 at 9:01pm
2.Best keep clear of the flapping sheets of wood then:) Cheers for the clarification Rog.
 
1. I used Bassbox 4.0 back in the mid to late 90's to build my 4 subs and thought it was god, buggar it. The cabs do seem to perform well, and seem to at least double the output over their bandwidth, so maybe they poke out 100dB instead of the drivers std 96dB. Def some good efficiency gains if not 11dB's worth.
 
So what now? I was planning on updating things and liked the look of the X1 or the C Sub. It appears both are 6th order designs, however I wondered why the X1's driver is mounted angled? I saw in another thread that the cone gets a hard time, so can it be realigned horizontal. I would imagine that the air resistance acting at an angle when box fully loaded at its higher tuned frequency would possibly pay havoc to the cone. Does the C Sub suffer from this? Just a semi-educated guess, any comments?
 
Anyway I have 4 x 18" RCF L18P200N  Fs-28Hz, Qms-7.15, Vas-363litres, Qts-0.31, Xmax-8mm.
 
Can I use bassbox pro to test these drivers in the C-sub and X1 bins? Or even better has anyone tested/simulated these drivers inside either of these cabinets?
 
Thanks for your help so far, and what an excellent site! Truly rapt to find this knowledge and info sharing.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote FlipC Quote  Post ReplyReply Direct Link To This Post Posted: 20 March 2009 at 11:17pm
Rog
Can you go into the directional aspects in more detail?
Having built numerous types of cabinets I have found that bass can be directed somewhat. 
The above BP is typical with omni directional radiation.
Where as with a correctly designed horn loaded sub there wont be much rear radiation. Of course with 40/50 Hz and up.  I have also noticed throw isn't strictly SPL related.  But I would think the difference between say some reflex boxes doing mid bass vs horns doing sub bass is because of frequency and human hearing.







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